280 THE PINEAL ORGAN 



accompanying thickening of its wall, and (b) a distal part, the structure of 

 which resembles and suggests that of the retina of the parietal eye. 



Lacertidte. 



The different species of Lacerta have been studied by Spencer, 

 Owsjannikow, Strahl and Martin, Leydig and Nowikoff. All the typical 

 parts of the pineal system are represented in the ordinary lizards, and 

 the variations in form and structure which are met with are largely due 

 to age and the concomitant degeneration found in old specimens. The 

 different stages of development were described by Nowikoff in 1910, and 

 have already been alluded to (p. 242). They confirm Klinckowstroem's 

 observations on Iguana with respect to the separation of the parietal vesicle 

 by constriction and the independent origin of the parietal nerve subse- 

 quent to the separation of the parietal vesicle. 



The pineal system of the adult lizard comprises the following parts, 

 which, passing from the superficial surface of the head towards the brain, 

 are : 



A pentagonal parietal scale, near the centre of which is a slightly 

 raised dark spot, the " parietal spot " (Fig. 192, C, p. 274). 



A pigment-free, transparent parietal area, which is formed of a 

 mucoid connective tissue, lying between the superficial closing 

 membrane of the parietal foramen and the epidermis. 

 The parietal eye and parietal nerve. 

 The terminal or habenular ganglion. 



A membranous fibrous capsule which encloses the retinal segment 

 of the parietal eye, and the parietal nerve ; in old specimens in which 

 the nerve-fibres have degenerated the fibrous sheath of the nerve 

 may persist as a cord uniting the parietal eye with the brain roof or 

 with the tip of the epiphysis. 



A pineal organ which consists of : a proximal part, springing from 

 the diencephalic roof between the habenular and posterior com- 

 missures ; a tubular body or stalk : an end vesicle, which is sometimes 

 prolonged forward as a fibrous cord which ends in the vicinity of the 

 parietal organ or is united with its fibrous sheath. 

 The end vesicle of the pineal organ is surrounded by blood-vessels 

 which are contained in a fibrous connective tissue which is sometimes 

 deeply pigmented. This is continuous with a similar vascular and 

 pigment-bearing connective tissue around the parietal organ. 



Striations have been observed in the peduncle of the proximal part 

 of the pineal organ, which Leydig considered were nerve-fibres, but no 

 separate pineal tract, such as that figured by Klinckowstroem in Iguana 

 tuberculata, appears to have been described. 



